Study On Surface Reaction Of Propylene Ammoxidation Catalyst Based On In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy And Monte Carlo Simulation

The oxidation and ammoxidation of propylene over bismuth molybdate catalyst was investigated in detail using in situ DRIFTS, molecule probe and dynamic RWMC simulation. Attention was particularly focused on the mechanism of propylene oxidation and ammoxidation, including the complete reaction networks of propylene oxidation, the route of acrylonitrile formation, the formation route of acrylonitrile from acrolein, and the transformation to acetonitrile from acetaldehyde. Conceptual design about propylene ammoxidation catalysts was proposed associated with dynamic simulation of propylene ammoxidation over bismuth molybdate catalyst. Such results were obtained as follows:The in situ DRIFTS equipment was successfully constructed with application to heterogeneous catalytic system, and in situ study on the surface reaction mechanism of propylene oxidation and ammoxidation was carried out. The results of in situ DRIFTS on propylene oxidation show that the allyl species originated from α-H abstraction of propylene were located at the wavenumbers of 1454 cm^-1 and 1427 cm^-1. The rate-determining step is the abstraction of an α-H abstraction from a lattice oxygen linked to a bismuth ion to form a Ï€-allyl intermediate coordinated to a molybdenum ion. The metal-oxo group then attacks the allyl intermediate forming a σ-bonded oxygen-allyl species, which is in a rapid equilibrium with the Ï€- bonded species. The σ-bonded species then transforms to acrolein by further abstraction of α-H. Such species as formate, carboxylate, carbonate transform to deep oxidation products (CO_x). The acetone was derived from propylene by an enolic species at lower temperatures.Based on in situ DRIFTS of propylene ammoxidation, four kinds of ammonia species on catalyst surface include: (i) ammonia dissociate adsorption with the formation of surface NH₂ species (1558 cm^-1) can take place, (ii) complete proton transfer can occur with the formation of an NH₄⁺ ion (1651 cm^-1 and 1435 cm^-1) on Bronsted acid sites, (iii) ammonia coordination (1246 cm^-1) on Lewis acid sites,which plays a crucial role in the formation of acrylonitrile from acrolein with partial oxidation of propylene, (iv) molybdenum-imido species (Mo=NH) (1029 cm^-1) are possibly proposed as the intermediates during the formation of acrylonitrile from the direct ammoxidation of propylene, which provides IR evidences of the presence of Mo=NH. The propylene molecule transforms to formaldehyde and acetaldehyde by degradation, the latter occurs to addition reaction with coordinated ammonia species, and forms acetonitrile by dehydration and oxidation.It is determined that the key active phase exists on the surface of propylene ammoxidation catalyst. The physical model of catalyst surface is built by means of binary digit technique, and the mechanism model of propylene ammoxidation is built. Thereby, the RWMC model applied to simulate the dynamic process of propylene oxidation and ammoxidation is constructed using Matlab. The simulation results show that the proper transfer rate of lattice oxygen is favorable for the reaction process, and the propylene conversion reaches the maximum after the reduction of catalyst to a certain degree. The transfer rate of lattice oxygen to the Mo vacancy differs from that to the O_A vacancy, and the latter is more rapid than the former. The probability of oxygen dissociative chemisorption contributes most favorably to the formation of acrylonitrile. Therefore, the improved way of propylene ammoxidation catalyst is proposed above the simulation results. It is inferred that catalytic performance improves greatly if the ratio of capacity for dehydrogenation from adsorbed propylene molecule on catalytically active site of molybdenum metal-imido group (Mo=NH) to that on catalytically active site of molybdenum metal-oxo group (Mo=O) becomes much higher. Another way is to optimize the bulk structure of catalyst to achieve the target of more lattice oxygen originated rapidly from gas oxygen and to speed up the transfer of lattice oxygen to the vacancies of Mo and O_A site.The conceptual design of propylene ammoxidation catalyst is proposed associated with the experimental and simulated results of this paper under the guide of site isolation and phase cooperation. Firstly, plenty of molybdenum-oxospecies on catalyst surface is obtained to make gas oxygen transform favorably into lattice oxygen. Secondly, enough molybdenum-imido species is derived from the lattice imido species transformed favorably from ammonia. Finally, it is fitting for the number and proportion of molybdenum-oxo species and molybdenum-imido species to form abundant oxo- molybdenum-imido species on catalyst surface. The actualized means of optimal design about propylene ammoxidation catalyst are suggested on the basis of this conceptual design. The yield of acrylonitrile is improved to optimize catalyst structure using site isolation. Single phase is designed based on phase cooperation, i.e., to integrate such multi-functionalities as hydrogen abstraction, oxygen-insertion (or nitrogen-insertion) into single phase. The acrylonitrile yield is enhanced to reduce the deep oxidation products and by-products originated from degradation reaction by designing catalyst for ammonia activated at low temperature. Finally, corresponding ideas of optimal design on propylene ammoxidation reactor and optimal controlling to reaction process are presented.